Same cluster as the early 240 through '80. I have had mine out and apart enough to fix the very problem you are speaking of. Unfortunately I can't give you a step-by-step without taking the cluster out and taking it all apart again. That was last year and I didn't take notes. But it's pretty obvious when you get in there.

I do recall a couple of things to watch out for.

Before attempting to remove the needle, pry the pointing end of it up just enough to get it over the peg (CCW, obviously) and take a pic or make a mark where it comes to rest. When you reassemble it install the needle pointing to the mark and the pry it back over the peg to the normal resting position.
To remove the needle I used the suggestion from somewhere on this site to use two spoons, one on either side, to gently pry it off. It works. But I would also suggest placing a bit of corrugated cardboard (or something of the like) under the spoons to protect the faceplate because it can be deformed pretty easily. I would also suggest working with the unit in a sack when removing the needle because otherwise it is going to go flying across the room. I still haven't found the pieces of the one that I broke while practicing. Then I broke the one on the unit I was actually fixing. I ended up replacing it with the orange needle from a later unit.

The two small screws you mention can be a problem too. It is very easy to tear up the slots revealing the yellow metal that they're made from so if cosmetics is an issue use a small screwdriver and a light touch. Obviously I messed up a couple of them while doing this but I was able to use some from another unit which I removed without ruining them. Sometimes they refuse to come out at all. I tried drilling the head off of one and there was not enough left protruding out to be able to extract it. I ended up using another speedometer.

As for the offending gear, there are a couple of ways to go.

If broken teeth is the problem replacement is you're only choice. For an excellent quality replacement gear see http://www.odometergears.com/. Jeff says his repros are not subject to the same splitting that causes the originals to cease functioning. Better material.
Or if it is split along the axis see http://www.homestead.com/volvo2/Odometer.html for a method of repair. In fact look at this one in any case. Although the unit pictured is the later mechanical type it's very similar to what you have and it does give you the step-by-step for disassembly.

One more thing that you may already know but I'll mention in case you don't is, don't reset the trip meter while the car is moving. This is said to be the cause of broken teeth on these gears.
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'80 DL 2 dr